1. Field of the Invention
The present invention relates to a magnetooptical recording medium. More particularly, the present invention relates to a magnetooptical recording medium which comprises a coating type magnetic layer comprising a hexagonal system of ferrite particles in a binder.
2. Description of the Related Art
Recently, a magnetooptical recording medium which magnetically records signals with a semiconductor laser beam has been vigorously studied and developed as a high density recording media.
Hitherto, the magnetooptical recording medium utilizes, as a magnetic element, mostly an amorphous alloy comprising at least one rare earth metal and at least one transition metal such as Tb-Fe-Co alloy, and is prepared by vacuum depositing or spattering such an alloy on a surface of a transparent substrate to form a thin magnetic layer.
With the magnetooptical recording medium comprising the alloy, signals are recorded by irradiating the laser beam on the magnetic layer, heating the magnetic layer to a temperature near a Curie temperature or a compensation temperature to decrease a coercive force of the magnetic layer and reversing magnetization with a magnetic field. The recorded signals are read out by using change of a rotation angle of a plane of polarization of a light reflected at an area in which the magnetization is reversed.
Although the conventional magnetooptical recording medium has an advantage that the amorphous alloy which constitutes the magnetic layer is highly sensitive to the laser beam, the alloy is easily oxidized so that the recording characteristics disadvantageously decrease as time passes. To prevent the oxidation of the alloy, various protective layers are formed on the magnetic layer. However, the oxidation of the amorphous alloy is not completely prevented.
In the conventional magnetooptical recording medium, a polarization angle of light, namely a Kerr rotation angle, is at most about 0.3 degree, so that a reproducing output is insufficiently low. It is proposed to increase the Kerr rotation angle by forming a dielectric layer of SiO.sub.x or SiN.sub.x on the magnetic layer and passing the light through the dielectric layer so as to use multiple interference. When the dielectric layer is formed, the Kerr rotation angle increases, while a reflectance of light decreases. Therefore, the dielectric layer does not greatly contribute to the improvement of recording characteristics.
Under such circumstances, it is suggested to produce a magnetooptical recording medium comprising a thin film magnetic layer which comprises an oxide such as garnet or a hexagonal system ferrite as a magnetic element and is formed on the transparent substrate. Such a magnetooptical recording medium does not suffer from the oxidation problem since the magnetic element is the oxide. But, since the oxide contains plural element different from the metal, it is difficult to keep a composition of the layer constant and thin films having the same characteristics are not prepared with good reproducibility.
To overcome the problem of reproducibility of the above thin film of the oxide, it is proposed to form a coating type magnetic layer on the transparent substrate, which magnetic layer comprises an organic or inorganic binder and hexagonal system ferrite particles dispersed therein. However, with such a magnetooptical recording medium, the Kerr rotation angle is not so large and the reflectance of light on the magnetic layer surface is small, so that the reproducing output tends to be decreased.